The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle having a torque converter, and more particularly to an open-loop control system for controlling a lockup clutch of the torque converter.
A continuously variable transmission having the torque converter with the lockup clutch is known. The torque converter operates to multiply torque of an engine, so that the vehicle can be smoothly started. After the start of the vehicle, the torque converter is preferably locked up by a lockup system, thereby preventing a loss of power and improving fuel consumption. Thus, various control systems for controlling the lockup clutch have been proposed in order to effectively use such characteristics.
However, when the continuously variable transmission (CVT) is provided on the vehicle, the transmission ratio changes continuously and inertia mass of pulleys is large. Therefore, it is not always preferable to lockup the clutch after the start of the vehicle in consideration of operation torque of the lockup clutch. Namely, it is desirable to use torque multiplication effect of the torque converter under certain driving conditions, such as at acceleration of the vehicle, even after the start of the vehicle.
Japanese Patent Application Laid-Open 63-303259 discloses a control system of the lockup clutch where the clutch is locked as soon as a transmission ratio of a CVT starts to change. That is, in the system, the torque converter is operated only at the start of the vehicle before the transmission ratio changes.